The incident, in which no one was injured, also has raised questions about safety in a city with many large buildings — but no laws requiring their exterior walls to be inspected as they age. Experts say such problems are relatively rare, but could become more common as building standards change.

The collapse occurred about 1 p.m. Tuesday, when about 20 rows of brick more than 20 feet long fell from the seventh and eighth stories of the building onto a grassy area outside the building, said Brad Moss, a spokesman for the NIH Office of Research Facilities.

A private engineering contractor has since been hired to determine the "root cause" of the collapse and develop a "remediation plan," Moss said in an emailed statement.

"The NIH has taken precautions to ensure that all building occupants are protected from any potential future issues," Moss wrote in the statement. "Shelters were constructed over all doors and exits and fencing was installed to restrict access to the area until further analysis can be conducted."

The incident recalled a series of issues that troubled the 10-story, 500,000-square-foot Biomedical Research Center during its construction between 2004 and 2008, as outlined in internal records obtained and reported on by The Baltimore Sun in 2006.

The problems included vibrations in the building that threatened the research of National Institute on Aging scientists, who rely on precise measurements with pinpoint lasers and finely tuned microscopes to investigate diseases such as Alzheimer's and Parkinson's. Such measurements can be disrupted by even slight vibrations in the building.

The issues were substantial enough to stall the transfer of some scientists to the building in 2006 as officials sought to understand the cause and possible solutions to the problem. Other problems in the building, which also houses researchers with the National Institute on Drug Abuse, were also outlined, including a shaky retaining wall, disruptions to water service and a leaky roof.

The NIH building, which overlooks Interstate 895 north of Greektown, was originally scheduled to open in the fall of 2006 but was delayed two years as costs rose to $278 million, about 25 percent above initial estimates.

By the time the building was repaired and unveiled in June 2008, officials said that the problems had been corrected and that testing showed acceptable levels of vibration in the building.

On Thursday, officials remained unsure what had caused the facade to break apart, officials said.

Jessica Murray, a spokeswoman with Skanska USA Building Inc., the construction contractor for the building, said in an email that officials with the company, which has offices in New Jersey, just learned of the problem Thursday after being contacted by a reporter.

"We take this issue very seriously, and are working diligently with NIH, the engineers and trade contractors to investigate the cause," said Murray.

One factor that could have contributed is an insufficient number of redundancies built into the structural support of the building's exterior, said Jeff Erdly, a facade restoration contractor and co-chair of the facade inspection task group at ASTM International, which sets best-practice guidelines for facade maintenance and inspections across the country.

Erdly, whose Pennsylvania-based company Masonry Preservation Services helped restore the brick facade of Baltimore's Harbor Court Complex about a decade ago, said the problem of falling facades has increased in recent years as construction trends continue to shift toward lighter buildings with fewer redundancies in facade support structures.

Last year, there were about a dozen facade collapses on major buildings reported in large cities, Erdly said, but he estimates that another 50 occurred without being covered in the media. "This phenomenon is growing, and it's growing because of the push to build lighter buildings," he said.

Even as problems become more prevalent, Baltimore and other large cities have failed to adopt tougher laws requiring regular facade inspections, Erdly said.He believes this is in part because regulatory programs are expensive and unpopular among builders, and in part because of a misplaced trust people have in the architecture that shapes their city skylines.

"Over the years, our society has looked at buildings and said, 'Well, my God, it's a brick building. We shouldn't have to [inspect] that. It should last 100 years,'" Erdly said. "But that's not true."

Nationally, fewer than 20 big cities have what are known as "facade ordinances" in place, he said. Those that do — including New York and Chicago — usually put them in place after high-profile tragedies involving falling materials.

"Every facade law that I am aware of in the United States has a horrific death linked to it," Erdly said.

New buildings are fully inspected by the city and must be awarded a use and occupancy permit, but facades are not inspected after that time unless specific problems are reported, said Cheron Porter, a spokeswoman with the city housing department, which handles commercial and residential inspections.

Erdly said more regular inspection requirements would help protect citizens, especially from crumbling older buildings.

For such a recently built structure like the NIH building to lose brick under normal weather conditions, he said, represents a "horrible, unacceptable situation" with deadly potential.

NIH's Moss reiterated that officials are taking steps to ensure the building's facade is safe. "Once the structural engineers have completed their initial investigation, a schedule for future activities will be developed."